Infrared radiation detection circuitry having a constant bias voltage across the sensor

ABSTRACT

Electromagnetic radiation detection circuitry incorporating a sensor which is responsive to radiation in the infrared region of the electromagnetic spectrum. Variations in the detected radiation are sensed as a change in current conduction by the sensor and are strengthened by an amplifying network including an amplifying transistor. A constant bias voltage is maintained across the sensor by the action of a biasing network having a control transistor, which has its base direct coupled to the base of the amplifying transistor, and a balancing resistor, which has a resistance equal to the nominal resistance of the sensor and which in the biasing network simulates the sensor.

United States Patent [72] Inventor Clyde A. Boenke 3,278,746 10/1966Fiat 250/833 H Ann Arbor, Mich. 3,418,478 l2/1968 Falbel 250/833 H [21]Appl. No. 62,219 3,448,267 6/1969 Blythe et al 250/833 H [22] Flled July1970 Primary ExaminerArchie R. Borchelt [45] patented Attorneys-Fisher&Schmidt William F Thornton and [73] Assignee The Bendix CorporationFlame Hafiz smith and Thompson [54] INFRARED RADIATION DETECTIONCIRCUITRY A g g NT BIAS VOLTAGE ACROSS ABSTRACT: Electromagneticradiation detection circuitry 8 Claims 2 Drawing Figs incorporating asensor which is responsive to radiationin the infrared region of theelectromagnetic spectrum. Variations in U.S. H, the detected radiationare sensed as a change in current con. 250/ R duction by the Sensor andare strengthened by an amplifying [latnetwork including an transistor Aconstant Field Of R, voltage is maintained across the sensor the actionofa H ing network having a control transistor, which has its base directcoupled to the base of the amplifying transistor, and a [56] Referencescued balancing resistor, which has a resistance equal to the nominalUNITED STATES PATENTS resistance of the sensor and which in the biasingnetwork 3,144,554 8/1964 Whitney 250/833 H simulates the sensor.

PREA COLLECTING INFRARED M O TROL OPTICAL SENSOR 8- BIAS SYSTEMCIRCUITRY SYSTEM RECO R D I N G OPTICAL SYS T EM "immune-m J 3.614.441

l2 RECO RD NG S OPTICAL SYSTEM f f 18 I I 1 COLLECT'NG INFRARED PREAMPCONTROL OPTICAL SENSOR & BIAS SYST EM SYSTEM CIRCUITRY I "IO CONTROLSYSTEM INVILINI'OR. v

ATTORNEYS INFRARED RADIATION DETECTION CIRCUITRY HAVING A CONSTANT BIASVOLTAGE ACROSS THE SENSOR This invention relates to electromagneticradiation detection circuitry adapted, although not exclusively, tosense radiation in the infrared region of the electromagnetic spectrumand to develop amplified and usable output signals reflecting thisinfrared radiation.

Whenever low-resistance sensors, such as those commonly employed tosense infrared radiation, are to be utilized, by way of example only,for direct photographic imaging by infrared imagery or as it will behereinafter referred to, by thermal mapping, only a small change in theconductance of the sensor occurs when the usual thermal groundcharacteristics are sensed. The corresponding small changes in currentflow through the sensor are difficult to measure and amplification istherefore required. Necessarily, the amplification must be done over arelatively wide bandwidth and while generating a minimum noise in orderto obtain a usable output signal. Capacitive couplings are generallyundesirable because DC and slowly varying signal changes cannot becoupled or passed. Other concerns are proper bias for the sensor andprotection of the sensor against burnout during operating extremes.

With the foregoing in mind, new and different circuitry is contemplatedfor amplifying changes in a certain characteristic of a low-impedancesensor.

Also contemplated is circuitry for maintaining in a unique way arelatively constant bias voltage across a low-impedance sensor.

Further contemplated is detection circuitry having amplifying andbiasing networks uniquely integrated to provide amplification oflow-level signals and the proper operating bias.

A more specific objective is to combine with an amplfying transistorhaving an infrared sensor in the input thereof a biasing network whichby simulating the current flows in the input and the output of theamplifying transistor establishes a relatively constant bias voltageacross the sensor. Other objectives include the provision of circuitryfor direct coupling a low-impedance sensor with an amplifier; circuitryaffording a direct coupling between an amplifier having a lowresistancesensor in the input and a biasing network which provides operating biasboth for the amplifier and the low-resistance sensor;

circuitry including an amplifying network capable of strengthening alow-level signal with a minimum of noise generation; and circuitry foramplifying changes in the operating characteristics of a low-resistancesensor and including a built-in provision for protecting the sensor fromdamage from operating extremes.

The foregoing and other objects and advantages of the invention becomeapparent from the following description and from the accompanyingdrawings, in which:

FIG. I is a block diagram of thermal-mapping apparatus incorporatingprinciples of the invention; and

FiG. 2 is a diagram of detection circuitry for the FIG. 1 system anddepicts amplifying and biasing networks for strengthening signalsderived from an infrared sensor employed in the FIG. 1 system, all beingconstructed according to the invention.

Referring to the details of the drawing and first to FIG. 1, theapparatus portrayed is for direct photographic imaging in the nonvisibleregion of the electromagnetic spectrum.

With this apparatus, the infrared radiation emitted by the earthssurface is used to obtain a thermal image of the earth's surface and istranslated into varying shades of gray on photographic film. Thisapparatus can be used for many purposes,

such as nighttime mapping over enemy territory, and analyzing fire,water, and soil conditions.

This thermal-mapping apparatus is generally well-known and thereforewill be discussed only in sufficient detail to understand the principlesof the invention. Briefly then for thermal-mapping purposes, theapparatus will be suitably mounted on an aircraft and includes anappropriate collecting optical system It) capable of scanning a scene,such as that at 12, and transferring variations in the infraredradiation from the scene 12 to an electromagnetic radiation sensor;e.g., an infrared detection sensor, designated generally at 14. Theseinfrared radiation variations are converted to voltage variations andamplified by preamplifying and biasing circuitry denoted generally at16. A control system assigned the numeral I8 includes provision foradditional amplification and whatever processing is required for thesevoltage variations and for converting them to visual equivalents. Forinstance, these voltage variations can be fed through an appropriateglow modulator driver, such as a high-voltage power transistor, and tosome suitable type of high-frequency response glow modulator; e.g., aSylvania R1168 gaseous discharge crator tube. In this way infraredradiation variations are changed to radiation variations in the visiblewavelength region of the spectrum. The image from the glow modulator arcis then reproduced after transfer through a recording optical system 20,which includes appropriate mirrors, as densityvariations in the filmforming part of a film cassette 22.

Considering now the FIG. 2 circuitry, the infrared sensor 14 isillustrated or a variable impedance and can be of any known type. In theFIG. 1 system a cryogenically cooled semiconductor, sensor 14 isemployed. The semiconductor material was mercury-cadmium-telluride.Cryogenic operation was at 77 K. and was provided by a liquid nitrogencoolant and a vacuum insulated dewar. In operation, the resistance ofthe infrared sensor 14 decreases with an increase in the temperature,which occurs when the sensed infrared radiation increases. When thesensors resistance decreases, there is an increase in current flowthrough the sensor 14.

Characteristically, an infrared sensor has a fixed high-conductance;e.g., 50/ 1,000 of an mho, which only changes a small amount; by way ofexample, one micro-mho, as thermal ground characteristics change.Therefore, as can be appreciated, it is difficult to measure thesechanges without amplification, which is a problem because the noisegenerated by the usual amplifier renders any amplified signal unusable.Also, if an instantaneous measurement is to be made to detect an abruptterrain change as when flying from land to water, any kind of capacitivecoupling would average this change and render it nonrepresentative ofthe crossover. Furthermore because of the low-resistance aspect of thesensor I4, current surges cannot be tolerated, since they wouldirreparably damage the sensor 14. The FIG. 2 circuitry overcomes theseproblems and others by combining an amplifying network 24 and a biasingnetwork 26, which together facilitate the strengthening of these smallcurrent variations developed by the infrared sensor l4 into usableoutput signals.

Continuing to refer to the FIG. 2 circuitry and considering first theamplifying network 24, there is included an amplifying device, such asan NPN-type transistor 28, which operates as a class A amplifier and isarranged in a common base configuration. In this configuration, abase-emitter input circuit 30 has the infrared sensor 14 connectedbetween ground at 32 and the emitter of the transistor 28 and abase-collector output circuit 34 with a load resistor 36 connected tothe collector of the transistor 28. Current flow changes through theload resistor 36 result in corresponding voltage variations that are fedas output signals to the control system 18. The collector of hetransistor 28 is connected to a positive voltage source at 38, whereasthe emitter of the transistor 28 is connected through a relatively highresistance 40 to a negative voltage source at 42. The high resistance 40and the negative voltage source at 42 combine to provide, in effect, aconstant current source. By way of example and without limitation, thisresistance can be 1,800 ohms, the negative and positive voltages 7.5volts and the transistor 28 can be type 2N440l.

The biasing network 26, which provides the operating bias for thetransistor 28 and also for the infrared sensor 14, includes a controldevice, such as a NPN-type transistor 44 connected as a diode. Thecontrol transistor 44 is selected so as to have the same characteristicsas the amplifying transistor 28 since the biasing network 26 is tosimulate all of the various currents in the amplifying network 24. Forthis reason, the voltages and resistances used by amplifying network 24are duplicated and a substantially identical transistor is employed.Hence, the control transistor 44 has a balancing resistor 46 with aresistance equal to the nominal resistance of the infrared sensor 14connected between its emitter and the ground at 32. The controltransistor 44 also has its emitter connected to a constant currentsource similar to that provided by the high resistance 40 and thenegative voltage source at 42. This constant current is provided by arelatively high resistance 48 and a negative voltage source denoted at50. To complete the simulation the collector of the control transistor44 is connected through a resistance 52, which has a value equal to thatof the load resistor 36, to a positive voltage source at 54. Then torender the control transistor 44 operative as a diode, a shunt 56connected between its collector and base.

To complete the circuitry the networks 24 and 26 are direct coupled.This is done by joining together as the bases of the amplifyingtransistor 28 and the control transistor 44.

The current flow through the control transistor 44 and the balancingresistor 46 will, in operation, generate noise currents and voltagesover a broad frequency spectrum which, as can be appreciated, aredetrimental when such weak signals as those derived from the sensor 14are to be amplified. For this purpose, a filter capacitor 58 isconnected between the bases of the transistors 28 and 44 and the groundat 32 and will operate to bypass to ground all of the high-frequencynoise contributions of transistor 44 and its associated resistors and aportion of the high-frequency noise produced by transistor 28.

The NPN-type transistors 28 and 44 and polarities shown are fordemonstration purposes. PNP-type transistors can, of course, be usedwith opposite polarities, if preferred.

In operation, the FIG. 2 circuitry will be assumed for convenience tohave the current flows and the directions indicated by the arrows. Asmentioned, the biasing network 26 since it has the same circuitparameters as the amplifying network 24 and also the control transistor44 has the same characteristics as the amplifying transistor 28, thesecurrent flows will be the same. For instance, the collector currents land l, and the emitter currents I and l i which, respectively, representthe current flows for the control transistor 44 and the amplifyingtransistor 28 will be the same, except the collector current l willinclude the current changes generated by the resistance variations ofthe infrared sensor 14 when operating in a sensing mode.

initially, the emitter of the control transistor 44 will be slightlynegative, and hence, it will commence conduction. The collector currentl will split with part proceeding by way of the shunt 56 to both of thebases of the two transistors 44 and 28. The Beta factor of thetransistor 44 will determine the relative amounts of these currents andis selected accordingly. The base currents, of course, will be small andthey will preferably be substantially equal. The current I, will be madeup of the portion of the collector current 1 not shunted, the basecurrent, and the current flowing through the balancing resistor 46.Being mindful that the two transistors 28 and 44 have the samecharacteristics they will each, therefore, have the same base to emittervoltage drop. With base current now being supplied to the amplifyingtransistor 28, the amplifying transistor 28 will commence to conduct.Since the emitter currents will be substantially the same, the voltageimposed across the infrared sensor 14 will be the same as the voltageacross the balancing resistor 46 and, of course, will be substantiallyconstant. It should be noted that the amplifying transistor 28 willbecome conductive when the voltage across the balancing resistor 46 andthe voltage across the infrared sensor 14 are approximately equal. Atthis time the two transistors 28 and 24 have the same base voltages, thesame base to emitter voltage drops and also the same emitter voltagessince they have each the desired same emitter currents.

When the infrared detector 14 is exposed; e.g., to increased infraredradiation, its resistance will decrease and there will be acorresponding current increase, which as has been discussed, isvirtually impossible to measure because the resultant changes in theconductance changes of the infrared sensor 14 are very small. By havinga constant voltage across the infrared sensor 14 and with the constantcurrent source presented by the substantially large resistance 40 andthe negative voltage source 42 and the current I, will remainsubstantially constant and is made-up of the collector current I thecurrent through the infrared sensor 14 and the base current. Since thecollector current 1 is much larger than the base current, it willcontain most of the variations sensed by the infrared sensor 14.Consequently, the signal fed to the control system 18 and taken acrossthe load resistor 36 will represent amplified versions of thesevariations in the sensor current.

The foregoing describes the normal operation of the FIG. 2 circuitry.If, however, the temperature, of the infrared sensor 14 increases to apoint where there is substantially a zero impedance or resistance, aswould be the case with depletion of cryogenic coolant, the emitter ofthe amplifying transistor 28 will, in effect, be connected to the groundat 32; then, the amplifying transistor 28, which requires a slightlynegative emitter voltage, will turn off since the base to emittervoltage for the transistor 28 is inadequate to maintain the amplifyingtransistor 28 on. Therefore, substantially all of the current 1 will gothrough the infrared sensor 14 and is selected in accordance with theoperating characteristics of the infrared sensor 14 so as to not damagethe sensor 14. Hence, the current through the infrared sensor 14 islimited to the selected safe current value.

From the foregoing, it will be appreciated that by this FIG. 2circuitry, a constant voltage is maintained across the infrared sensor14 so that the small variations in the impedance of the infrared sensor14 can be converted to a usable amplified signal. This is all donewithout capacitive couplings and without interference from the noisegenerated by the control transistor 44 and the balancing resistor 46.Also, as those versed in the art will appreciate; the principles of theinvention can be applied to other than thermal mapping and are notrestricted to infrared sensors but can be utilized with any kind ofsensor of its equivalent.

What is claimed is:

1. Circuitry comprising sensing means operative to vary a currentconducting characteristic thereof in accordance with a sensed property,means amplifying changes in the current conduction by the sensing means,and biasing means maintaining a certain relatively constant voltageacross the sensing means, the biasing means including a balancingelement having a current conducting characteristic substantially thesame as the current conducting characteristic of the sensing means, andbeing operative to simulate the current flows through the amplifyingmeans so as to establish the certain relatively constant voltage acrossthe balancing element and also being operative to connect the balancingelement in circuit with the sensing means so that the certain relativelyconstant voltage is maintained across the sensing means.

2. Circuitry as described in claim 1, wherein the amplifying meansincludes an amplifying device having an input circuit including thesensing means and an output circuit connected to a load and the biasingmeans simulates the current flow in the amplifying device 's input andoutput circuits.

3. Circuitry as described in claim 2, wherein the biasing means furtherincludes a control device operative to establish a predetermined currentflow through the balancing element so as to develop the certainrelatively constant voltage thereacross.

4. Circuitry comprising means sensing electromagnetic radiation andhaving a certain impedance value, the sensing means being operative tovary the impedance thereof in accordance with the sensed radiation andcorrespondingly the current conduction thereby, means amplifying thechanges in the current conduction by the sensing means, the amplifyingmeans including an amplifying device having an input circuit includingthe sensing means an output circuit connected to a load, biasing meansmaintaining a certain constant voltage across the sensing means, thebiasing means including a balancing element having an impedancesubstantially equal to the certain impedance value of the sensing meansand a control device operative to establish a current flow through thebalancing element so as to develop the certain relatively constantvoltage thereacross and also operative to connect the balancing elementin circuit with the sensing means so that the certain relativelyconstant voltage is also maintained across the sensing means.

5. Circuitry as described in claim 4, and further including filteringmeans arranged in circuit with the biasing means so as to attenuatenoise producing frequencies generated thereby.

6. Circuitry comprising means providing a constant current; meansproviding a constant voltage; an infrared radiation sensor constructedand arranged so that the resistance thereof varies from a certain valuewith the infrared radiation sensed thereby and correspondingly thecurrent conduction thereby; means amplifying changes in the currentconduction by the sensor; the amplifying means including am amplifyingtransistor having emitter, collector and base electrodes, the amplifyingtransistor having the input base-emitter current thereof arranged sothat the sensor and the constant current means are connected in paralleltherein and the output basecollector circuit thereof is connected to theconstant voltage means through a load; biasing means maintaining acertain relatively constant voltage across the sensor; the biasing meansincluding a balancing element having a resistance substantially equal tothe certain resistance value of the sensor, a control transistor havingbase, emitter, and base electrodes and arranged so as to have the baseelectrode thereof connected to the base electrode of the amplifyingtransistor, the emitter electrode thereof arranged to connect inparallel the balancing element and the constant current means and thecollector electrode thereof connected to the constant voltage means, anda shunt connection between the collector and base electrodes, thecontrol device being operative to establish current flows in the biasingmeans corresponding the current flows in the amplifying means so as todevelop the certain relatively constant voltage across the balancingelement and to connect the balancing element in circuit with the sensorso that the certain relatively constant voltage is also maintainedacross the sensor.

7. Circuitry as described in claim 6, wherein the infrared radiationsensor is constructed to pass a predetermined maximum current and theconstant current means is arranged to provide a constant current of avalue not exceeding the predetermined maximum current.

8. Circuitry as described in claim 7, and further including filtercapacitive means connected to the base electrodes of both the amplifyingtransistor and the control transistor so as to attenuate noise producingfrequencies generated by the biasing means.

1. Circuitry comprising sensing means operative to vary a currentconducting characteristic thereof in accordance with a sensed property,means amplifying changes in the current conduction by the sensing means,and biasing means maintaining a certain relatively constant voltageacross the sensing means, the biasing means including a balancingelement having a current conducting characteristic substantially thesame as the current conducting characteristic of the sensing means, andbeing operative to simulate the current flows through the amplifyingmeans so as to establish the certain relatively constant voltage acrossthe balancing element and also being operative to connect the balancingelement in circuit with the sensing means so that the certain relativelyconstant voltage is maintained across the sensing means.
 2. Circuitry asdescribed in claim 1, wherein the amplifying means includes anamplifying device having an input circuit including the sensing meansand an output circuit connected to a load and the biasing meanssimulates the current flow in the amplifying device''s input and outputcircuits.
 3. Circuitry as described in claim 2, wherein the biasingmeans further includes a control device operative to establish apredetermined current flow through the balancing element so as todevelop the certain relatively constant voltage thereacross. 4.Circuitry comprising means sensing electromagnetic radiation and havinga certain impedance value, the sensing means being operative to vary theimpedance thereof in accordance with the sensed radiation andcorrespondingly the current conduction thereby, means amplifying thechanges in the current conduction by the sensing means, the amplifyingmeans including an amplifying device having an input circuit includingthe sensing means an output circuit connected to a load, biasing meansmaintaining a certain constant voltage across the sensing means, thebiasing means including a balancing element having an impedancesubstantially equal to the certain impedance value of the sensing meansand a control device operative to establish a current flow through thebalancing element so as to develop the certain relatively constantvoltage thereacross and also operative to connect the balancing elementin circuit with the sensing means so that the certain relativelyconstant voltage is also maintained across the sensing means. 5.Circuitry as described in claim 4, and further including filtering meansarranged in circuit with the biasing means so as to attenuate noiseproducing frequencies generated thereby.
 6. Circuitry comprising meansproviding a constant current; means providing a constant voltage; aninfrared radiation sensor constructed and arranged so that theresistance thereof varies from a certain value with the infraredradiation sensed thereby and correspondingly the current conductionthereby; means amplifying changes in the current conduction by thesensor; the amplifying means including am amplifying transistor havingemitter, collector and base electrodes, the amplifying transistor havingthe input base-emitter current thereof arranged so that the sensor andthe constant current means are connected in parallel therein and theoutput base-collector circuit thereof is connected to the constantvoltage means through a load; biasing means maintaininG a certainrelatively constant voltage across the sensor; the biasing meansincluding a balancing element having a resistance substantially equal tothe certain resistance value of the sensor, a control transistor havingbase, emitter, and base electrodes and arranged so as to have the baseelectrode thereof connected to the base electrode of the amplifyingtransistor, the emitter electrode thereof arranged to connect inparallel the balancing element and the constant current means and thecollector electrode thereof connected to the constant voltage means, anda shunt connection between the collector and base electrodes, thecontrol device being operative to establish current flows in the biasingmeans corresponding the current flows in the amplifying means so as todevelop the certain relatively constant voltage across the balancingelement and to connect the balancing element in circuit with the sensorso that the certain relatively constant voltage is also maintainedacross the sensor.
 7. Circuitry as described in claim 6, wherein theinfrared radiation sensor is constructed to pass a predetermined maximumcurrent and the constant current means is arranged to provide a constantcurrent of a value not exceeding the predetermined maximum current. 8.Circuitry as described in claim 7, and further including filtercapacitive means connected to the base electrodes of both the amplifyingtransistor and the control transistor so as to attenuate noise producingfrequencies generated by the biasing means.